“Evaluate the use of indirect taxation to correct the market failure arising from pollution.”

Diagnosis — name the failure An indirect tax is the government’s classic tool for a negative externality, so the answer must start with the market failure the tax is meant to cure. Pollution is a negative production externality: when a firm produces — a coal-fired power station, a chemical plant, a cattle ranch — it imposes costs on third parties who are no part of the transaction, whether that is residents inhaling toxic smoke, communities whose river is poisoned, or everyone living with a warmer planet. Because the firm considers only its private costs and ignores these external costs out of self-interest, the true marginal social cost lies above the marginal private cost by the value of the marginal external cost (MSC = MPC + MEC). The free market therefore settles at P1, Q1, where MPB = MPC, rather than at the social optimum P*, Q*, where MSC = MSB. The result is over-production and over-consumption: output is too high, the price is too low because it fails to reflect the true social cost, and there is a welfare loss on every unit produced beyond Q*, where the cost to society exceeds the benefit. Too many of society’s scarce resources are being poured into a polluting activity.

The mechanism — how the tax fixes it An indirect tax such as a carbon tax now corrects this. The tax raises the firm’s cost of production, which shifts the supply curve (the MPC) to the left, from MPC to MPC + tax. If the tax is set equal to the marginal external cost, the new MPC + tax curve coincides exactly with the MSC. This is the crucial step, and the phrase that unlocks the top band is that the externality has been internalised: pollution used to be free — it cost the firm nothing to dump waste into the river — but now the firm must pay a cost that mirrors the cost it imposes on society, so it behaves as if it bears the full social cost of its actions. As the higher cost is partly passed on to consumers, the price rises from P1 to P*, and the higher price rations demand so that quantity falls from Q1 to Q*. At Q* the market once again produces where MSC = MSB: the over-production is eliminated, the misallocation of resources is corrected, the welfare loss is recovered, and social welfare is maximised. The polluter, at last, pays.

Evaluation 1 — inelastic demand However, whether the tax actually works turns on the elasticity of demand, and for exactly the goods we most want to tax, demand tends to be price inelastic. Fossil fuels, tobacco, alcohol and sugar are necessities, or addictive, and have few close substitutes, so consumers keep buying them almost regardless of price. This matters because when demand is inelastic, a given tax raises the price a lot but reduces quantity only a little: the fall in quantity from Q1 towards Q* is proportionately smaller than the rise in price. As a result the reduction in output — and therefore the reduction in pollution — is disappointingly small, so the over-production and the welfare loss are only partly corrected rather than eliminated. To cut pollution meaningfully the government would have to set an extremely high tax, which then intensifies every other problem below. The policy still moves the market in the right direction, but on its own it may not reach the social optimum at all.

Evaluation 2 — regressivity, the black market and government failure A second problem is that indirect taxes are regressive: they take a larger proportion of income from low-income households than from high-income households, so a flat tax on petrol or cigarettes proportionally hurts people on low incomes more, which widens inequality — and the effect is amplified precisely because demand is inelastic, since the poor cannot easily cut back. Worse, a high tax invites an unintended consequence: because roughly half the price of a packet of cigarettes is already tax, there is a powerful incentive to evade it, and consumers turn to a black market that pays no tax at all. The UK already has an estimated £2bn illicit-tobacco market; when the United States banned alcohol under Prohibition, an entire moonshine economy sprang up and people went blind on contaminated spirits. A black market carries five distinct costs — policing, lost tax revenue, consumers not knowing the quality of what they buy, price exploitation by criminal suppliers, and smuggling — and it deepens rather than corrects the misallocation of resources. If these costs, together with the tax’s administration and enforcement costs, outweigh the benefit of the pollution it prevents, then the intervention has produced a net welfare loss: this is government failure, where the cost of government action exceeds its benefit, and the policy should not have been used at that level at all.

Judgement On balance, an indirect tax is a justified and often sensible response to pollution, but it is rarely a complete cure, and how well it works depends on three things: the elasticity of demand, the government’s ability to measure the externality, and how high the tax is pushed before evasion and shutdowns set in. Its great virtue is that, unlike an outright ban, it allows trade and the benefits from trade to continue — and that matters, because most polluting activities carry large benefits, especially on the first units: ban all pollution and you ban the lorries that deliver food to the supermarkets, and people starve pretty quickly. The tax keeps the valuable output and prices only the harm. My judgement is therefore that indirect taxation should be the first-best instrument where the external cost can be measured with reasonable confidence and demand is at least moderately elastic — the sugar levy is a defensible example — but that for stubbornly inelastic, hard-to-measure pollution it works best in combination with other tools such as tradable permits and targeted regulation, rather than being asked to do the whole job alone.